An MRI device is a device with which all or part of a patient's body is inserted into the air gap of a magnetic field generator that forms a powerful magnetic field, and a tomographic image is obtained so that the tissue thereof can be evaluated.
This MRI magnetic field generator has to be wide enough for all or part of a patient's body to be inserted, and to obtain a sharp tomographic image it is usually necessary to form a stable, powerful, uniform magnetic field having a precision of 1.times.10.sup.-4 or less and 0.02 to 2.0 T within the imaging field of view in the air gap.
One known magnetic field generator used in MRI devices has a structure in which magnetic pole pieces are fixed facing each other at one end of each of a pair of permanent magnet structures in which an R--Fe--B-based magnet is used as the magnetic field generation source, the other ends are coupled by a yoke, and a static magnetic field is generated within an air gap between the magnetic pole pieces (Japanese Patent Publication 2-23010).
Another known structure has electromagnetic coils (including normal conduction coils, superconduction coils, and so on) wound around iron cores for the magnetic field generation source in place of the above-mentioned permanent magnet structures (Japanese Laid-Open Patent Application 4-288137), and the same magnetic pole pieces as those mentioned above are also employed in this structure.
As mentioned above, the uniformity of the magnetic field of a magnetic field generator needs to have a precision of 1.times.10.sup.-4 or less within the specified space, but this is greatly affected by the shape of the magnetic pole pieces, particularly in the magnetic circuit, and is also affected by the shape of the yokes, where the device is installed, and other such factors, making it inevitable that various adjustments will need to be made in order to bring the magnetic field uniformity within the imaging field of view to the specified value. Therefore, a structure had to provide a magnetic field of extremely high uniformity and allow the magnetic field intensity within the required air gap to be locally increased or decreased by the required amount without the magnetic pole pieces being re-machined.
In view of this, the applicant, assuming the imaging space to be a sphere, proposed that the magnetic field intensity within the air gap could be locally increased or decreased by the required amount and the magnetic field uniformity thereby increased without the magnetic pole pieces having to be re-machined if magnetic material pellets[1] and/or permanent magnet pellets were disposed at specific locations on the sides of the magnetic pole pieces facing the air gap according to the magnetic field intensity at various circumferences measured by traversing this spherical space in a plurality of horizontal planes (Japanese Patent Publication 5-87962).
To obtain position data within the air gap with an MRI device, gradient coils (GC) consisting of a set of three coils corresponding to the X, Y, and Z directions are usually disposed in the vicinity of the magnetic pole pieces, and a gradient field can be generated in the desired direction within the air gap by applying a pulse current to these gradient coils. In other words, position data is given to the nuclear magnetic resonance signal by adding a gradient field to the uniform magnetic field formed within the air gap, and a large number of pulsed gradient fields must be added in order to obtain a single image.
Meanwhile, when the magnetic field is altered by the effect of the environment where this magnetic field generator has been installed, for example, or when the MRI device has already been installed at its usage site, once the gradient coils have been installed on the magnetic pole pieces, adjusting the magnetic field uniformity by placing and moving the magnetic material pellets and/or permanent magnet pellets as discussed above is difficult and complicated work, and this work cannot be completed in a short time.
Japanese Laid-Open Patent Application 1-164356 proposes a method for arranging field adjusting permanent magnet pellets for the purpose of adjusting the magnetic field uniformity, wherein a plurality of field adjusting permanent magnet pellets are fixed by screws to a support plate via pedestal, and this support plate is bolted to an annular protrusion on the magnetic pole pieces with the permanent magnet pellets on the magnetic pole piece side, or [the permanent magnet pellets] are disposed with respect to the air gap from the outer peripheral side of the pair of magnetic pole pieces. The problem with this structure is that removing the shims takes a very long time because the permanent magnet pellets are fixed by screws to the support plate via a pedestal.
Specifically, magnetic material pellets or permanent magnet pellets that are used for adjusting the magnetic field uniformity are not all of the same size and shape. First, a selection is made between a magnetic material and a permanent magnet according to the amount of adjustment or the position of the space for adjustment, and then the shape and size thereof are suitably selected, so a pedestal must be fabricated according to the size and shape of the magnetic material for [the magnetic material] to be fixed by screws to the support plate via the pedestal, and a pedestal of the required shape must be fabricated according to the size and shape of the permanent magnet pellets so that the magnetization thereof will face in the proper direction, so removal of the shims entails a tremendous amount of work including pedestal fabrication.